debug/readlink{, at}_chk.c: Harmonize declaration and definition

The declaration and definition of these routines aren't consistent.

Make the definition of __readlink_chk and __readlinkat_chk match the
declaration of the routines they fortify.  While there are no problems
today this avoids any future potential problems related to the mismatch.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>